I wrote a bunch a while back on this, having helped on the Fantail conversion for the S-76 when we were demonstrating in the LHX competition.
The fan/fenestron is never as efficient as a tail rotor (nor is a notar, BTW), it only barely breaks even power-wise. Remember every wasted HP is about 5 to 7 pounds of payload you left behind. Also, the weight and drag of the fan is always worse than a tail rotor. The shroud alone is heavy, it must fit perfectly, and must not warp when you whip the aircraft around, because tip clearances beyond a fraction of an inch waste lots of power, but shroud contact is a real no-no. The drag of the fan (the shroud has to be deep enough to guide the air, so it is boxy) is about 15% of the total drag of the S-76 it was on!
Also, the fan does not want to have two gearboxes, so it must be on the shaft centerline. this means that the fan and shroud are real ground strike hazards, and therefore need even more beefing up to keep the shroud from deforming when the ground is struck, leading to a major blade strike problem if you whack the tail bumper.
Why did Comanche have a fan? Because the Army said, he who has an unprotected tail rotor automatically loses. We could read their lips.
Like NOTAR, Fenestrons are really design power and weight wasters, but they have a passionate following, and they sell a little, so what the heck!
BTW, the fan does not handle well at all, 212man, but the big endplates that fan equipped helos have do handle well. A naked fan is a pain in the neck in cruise flight, because the fan does not have any real yaw stability of its own (unlike a tail rotor, which behaves like a big, helpful fin).